Yes, depending on the raid configuration, this is possible or you will destroy everything. With a mirrored RAID you can do this step by step by first swapping a hard drive for an 8TB. As soon as the RAID is intact again, you can swap the smaller disc for an 8TB. This will not work with a striped RAID. If the disks are not set up as a RAID, you can swap them and copy the data if necessary.

It depends on the setup. Presumably (and sensibly) the hard drives run with RAID 1, i.e. the hard drives are mirrored. Then both hard drives must be the same size so that you can use the full capacity. Otherwise you will only have as much memory as the smaller of the two.

You can go to the "Storage Manager" and then to "Volume 1", then it should say "RAID type: Synology Hybrid RAID, 1-drive fault tolerance". This means that the hard drives are mirrored.